Switch to the form view to see all the extra fields, including date format.

Fill in the date format you want, give the language a unique name and code, and check the Translatable check box.

Click the save button.

Next, the users have to choose the new language. Either they can do it themselves through the User: Preferences menu item, or an administrator can do it for them through the Administration: Users: Users menu item.

Things that don't work in OpenERP 5.0.14:

Changing the date format on the default English language entry. It seems to get ignored, except maybe in reports.

Double clicking on a language entry doesn't bring up the form view, and neither does clicking the New button. You have to explicitly click the Form button.